I've been using a Reflectix wrap on my C14 since this spring.
My setup is a single layer that is wrapped around most of the OTA. The dovetail is exposed and the back of the telescope is exposed. The Reflectix wrap is significantly longer than the OTA, so it extends well past the corrector plate. Before wrapping the scope, I attach an AstroZap flexible dew shield which is about 1/2 longer than the Reflectix wrap. So I have no bare reflectix exposed to the optics on the inside, so it cannot produce reflections visible in the eyepiece. I also install a dew strap on the corrector end of the OTA, just behind the retaining ring that holds the corrector in place.
My usage scenario is that I travel to dark sky sites where I stay for multiple nights. I set up the scope on the first day and leave it set up until I leave the site. This can be anywhere from 4 nights to 2 weeks. I leave the scope covered during the day with a Telegizmos 365 cover. At dusk, once there is no direct sunlight hitting the OTA, I remove the cover, attach the dew shield and wrap the scope. I don't bother to cover the scope when I am done observing at night. I remove the wrap and the dewshield, and cover the scope, the next morning. This means that my scope tends to be near ambient temperature most of the time. I've not had any cases where I bring a scope from warm indoors to cold outdoors. There have, however, been a few cases where I've uncovered the scope before sunset, and the dark grey OTA heats very quickly when exposed to any sunlight.
My experience has been that there is a dramatic improvement in the scope's performance in the early evening. This past summer, we've had a nice display of planets early in the evening. I am highly confident that the Reflectix wrap significantly improved my planetary observing, versus not insulating the scope at all. The C14 mirror retains so much heat, that without some kind of mitigation for tube currents, it doesn't really settle down until many hours after sunset. In cases where the temperature drops all night, it may never settle down at all. This has not been a problem with the insulation. I have also noticed that dew is significantly reduced, but in some cases, I've still needed some heat to keep the corrector clear. (A C14 is among the world's best moisture collectors; we could potentially convert deserts to jungles just by setting up a bunch of C14s...)
As for visible light reflections, I don't have any Reflectix exposed to the scope's optics, as I mentioned above. And since I observe at dark sky sites, there has not been any problems with reflections off the external, exposed Reflectix. I have to say, however, that I don't think that covering or painting the exterior of the Reflectix would hurt it's performance at all in this application (but I've never tested it to be sure). We're trying to keep heat from radiating out of the aluminum OTA, and there is no significant source of radiant heat trying to enter the scope at night. I would say, though, that if you painted the Reflectix flat black and then exposed the scope to direct sunlight, you would probably make it heat up quicker than being unwrapped.
Those are my thoughts and experiences. My suggestion is that you try it for yourself. Reflectix is really cheap, and it's easy to wrap the scope. There's no significant cost in trying different variations to see what works best for you.